Rechargeable batteries are used in many applications to power a variety of devices. Different devices will discharge rechargeable batteries at different rates depending on the load being applied across the battery terminals. Each device may also discharge the rechargeable battery at different rates depending on the function being performed by the device. For example, a portable computer may discharge a rechargeable battery quickly when computing complex graphic calculations on a processor and rendering a graphic image on a display. The same portable computer may discharge the rechargeable battery more slowly when it is placed in "stand-by mode" and operation of the computer is temporarily suspended. Even when the portable computer is completely off, the rechargeable battery typically continues to discharge a small amount of current over time due to the internal resistance always present in the battery.
Typically, a rechargeable battery is charged with a transformer that converts current from a conventional electrical outlet or automobile lighter into direct current suitable for charging the battery. Once the rechargeable battery reaches a maximum voltage, it is fully charged. To protect both the rechargeable battery and the electronic device that it powers, it is important to carefully monitor and control both the charging and discharging processes. During the charge cycle, a battery can overheat and be destroyed if charged beyond the specified capacity of the battery. Overcharging can also harm the electronic device as well as people handling the device. In the discharge cycle, the electronic device may be damaged if a short develops within the battery or the device causing an sudden increase in current.
The device used to measure the charge/discharge state of a battery is popularly called a "gas gauge". Like the gas gauge on an automobile, the battery gas gauge measures how much charge is stored in a battery. Conventional gas gauges measure the current flow into and out of the rechargeable battery to measure the battery's charge. These conventional gas gauges detect the current flow into and out of the rechargeable battery using a fixed resistor that is coupled in series between the battery and the load. The voltage drop across the series resistor is directly proportional to the current flowing into or out of the rechargeable battery. Unfortunately, the series resistor, though typically very small in size, consumes a portion of the available power delivered by the rechargeable battery and cannot be used to accurately detect the wide range of currents drawn by many of the electronic devices. That is, the voltage drop produced by the very small series resistor may only be accurately detected when the current flow is high. If the current flow is low, the voltage drop across the very small resistor may be too small for most gas gauges to accurately detect. In order to increase the accuracy of the measurement, the size of the series resistor can be increased. However, increasing the size of the series resistor increases the power lost across the series resistor and, at high currents, further reduces the voltage available to the load. Consequently, conventional gas gauges may not accurately measure the charge state of the rechargeable battery. For example, a conventional gas gauge using a very small series resistor may only accurately detect the high current used when a computer is fully operational but may not accurately detect the lower current used when the computer is placed in "stand-by" mode.
Further, conventional gas gauges must integrate the current flow into and out of a battery over time to determine the total charge left in the battery. To make an accurate measure of the battery charge, a conventional gas gauge needs to accurately measure the elapsed time over several days or, in some cases, several months of battery usage. Keeping an accurate time basis may require additional circuitry and added complexity in the design of the gas gauge.